When a laser beam emitted from a laser device of an optical fiber laser module enters an optical fiber, part of the laser beam that has been incident on the optical fiber with an angle of incidence that is greater than the maximum acceptance angle of the optical fiber is not totally internally reflected at an interface between a core and a cladding, resulting in leakage light, which does not propagate within the optical fiber. Furthermore, when an optical axis of a laser beam is deviated, light may not enter a core of an optical fiber. Such light results in leakage light. The intensity of such leakage light increases as a laser has a higher output. Heat generated from the leakage light may damage a covering of an optical fiber, an adhesive for fixing the optical fiber, or the like.
Therefore, such leakage light should be treated in a proper manner. However, it is dangerous to emit leakage light directly to an outside of an optical fiber module. Thus, the leakage light should be converted into heat and then released into the outside of the optical fiber module. In a conventional optical fiber module, there has been proposed that, for example, a bottom of a fiber holder for holding an optical fiber is located on the same plane as a bottom of a base plate of a housing so that the fiber holder is brought into direct contact with a heat sink (see, e.g., Patent Literature 1). With this configuration, a heat dissipation path for heat generated in the fiber holder can be shortened. Therefore, the heat dissipation efficiency of the optical fiber module can be improved, and a temperature increase can be prevented near a portion of the optical fiber where a laser beam is optically coupled to the optical fiber (hereinafter referred to as a laser coupling portion). With this configuration, however, most part of leakage light is confined in the fiber holder and converted into heat therein. Recent development in increasing the power of an optical fiber laser module has caused an increased amount of heat generated in a fiber holder. Thus, generated heat may result in a temperature higher than the heat resistance temperature of an adhesive used for fixing an optical fiber to the fiber holder. Accordingly, the adhesive may be ignited or damaged by fire.
Furthermore, there has also been proposed to scatter a laser beam that has been incident on an optical fiber with an angle of incidence that is greater than the maximum acceptance angle of the optical fiber, with use of a light scattering member within a housing to release the leakage light as scattered light into an interior of the housing (see, e.g., Patent Literature 2). This configuration can reduce the power of light converted into heat near a laser coupling portion and prevent a temperature increase near the laser coupling portion. With this configuration, however, most part of light scattered in a forward direction from the light scattering member is directed to the inside of a fiber holder for holding an optical fiber and is thus converted into heat within the fiber holder. Accordingly, this configuration also suffers from the same problems as the configuration disclosed in Patent Literature 1.